432 



HISTORICAL GEOLOGY. 



The hexactinellid sponge Eujylectella (Venus's Flower-Basket), Fig. 29, page 57, which 

 looks as if made of a network of spun glass, conies from a depth of 50 fathoms in the East 

 Indies. The fossil Diclyophtjton and Euphantoiuia are related to Euplectella, as shown 

 by Whitfield. Sponges are mostly marine ; but a few, like the Spongillce, grow in fresh 

 ■water and contribute siliceous spicules to peat and other swamp deposits. The death and 

 decay of Sponges adds largely to the silica of the sea-bottom. 



446-460. 



Spongb-spicules. — Figs. 446-449, Geodia or allied; 450, Globostellate spicule, near Geodia ; 451, Stel- 

 letta; 452, Cai'terella; 453, 454, Tetractinellid spicules ; 455, Ventriculites, Hexactinellid; 456, Ragadinia 

 annulata; 457, Tisiphonia; 458, the same?; 459, Racodiscula; 460, Pliuthosella squamosa. Figs. 450, 453, 

 454 (xlO); 456 (x68); others (x34). Hinde. 



8. Protozoans. 



Among Protozoans only the Rhizopods and Eadiolarians have prominent importance. 



1. Rhizopods (Foraminifers) . — Species mostly minute, often forming shells; the 

 shells, with few exceptions, not larger than the head of a pin : but the gTOups sometimes 

 having the shape of disks an inch in diameter, and occasionally of large massive forms. 

 They have usually calcareous shells called Foraminifers (from foramen), and these have 

 contributed largely to the formation of limestone strata. They consist of 1 or more cells ; 

 and the compound kinds present various shapes, as illustrated in Figs. 461-474. The 

 arrangement in a group is usually alternate or spiral. Others make a shell or test by the 

 agglutination of grains of sand or other material. 



461-474. 



465/^ 466/ 



Figs. 461-474 — Rhizopods, much enlarged (excepting 473, 474). Fig. 461, Orbulina universa ; 462, Glo- 

 bigerlna rubra; 463, Textularia globulosa Ehr. ; 464, Rotalia globulosa ; 464 a, Side-view of Rotalia 

 Boucana ; 465, Graramostomum phyllodes Ehr. ; 466, a, Frondicularia annularis : 467, Triloculina Jose- 

 phina; 468, Nodosaria vulgaris; 469, Lituola nautiloides; 470, n, Flabellina nigosa ; 471, Chrysalidina 

 gradata; 472, «, Cuneolina pavonia ; 473, Nummulites nummularius ; 474 a, 6, Fusulina cylindrica. All 

 but the last two magnified 10 to 20 times. 



